As woofer Qts goes below 0.38, the alignment slides toward QB3, and as Qts goes above 0.38, it goes towards C4 alignments. Alignments BW4 and lower are free of ripple, while ripple increase as they move towards C4. The practical upper limit for driver Qts is around 0.45 to 0.5 (very large enclosures required, along with plenty of ripple).

The QB3 is somewhat misnamed: like all of the classical T/S alignments, it is a 4th-order highpass filter, but has a somewhat softer curve at the F3 frequency, and thus somewhat better group-delay characteristic (compared to BW4).

All vented systems have a zero in the woofer excursion at the box frequency; the depth of the zero is set by system losses - vent resistance, resistance in the driver suspension, and box leaks. In practical vented systems, the drop in woofer excursion is typically 10~20 dB, but as mentioned above, this is a narrowband reduction in excursion. At frequencies 1/2 octave or more below box frequency, excursion is (substantially) greater than the closed-box system, so the net excursion picture is a tradeoff. One of the more clever things a designer can do is set the box frequency very low, for example, 25 Hz or less, so the speaker is only rarely exposed to extreme LF input (which will increase in-band IM distortion and might damage the woofer).

This is true of all classical vented alignments, whether deep into QB3 territory, BW4, or C4. There are a number of higher-order alignments using active equalization at line level, with the best-known the BW6, using a 2nd-order highpass filter to synthesize a 6th-order highpass (first commercially realized in an Electro-Voice speaker in the mid-Seventies).

The majority of commercial vented loudspeakers are QB3's for the simple reason that modern (post-Seventies) woofers usually have Qts below 0.38, which has the side benefit of a smaller cabinet.

When I was at Audionics in the late Seventies, I preferred the sound of the Bessel 4th-order to the QB3. Back then, the computations were tedious on a handheld calculator, but is easy now. Part of the reason I may have preferred the sound is that the BL4 is less sensitive to woofer Qts variations than a QB3, at the expense of a somewhat higher F3. As you might imagine, the BL4 also has a bit better group delay compared to the the QB3.

The vent colorations of classical vented systems are an annoying problem for a 2-way system with a midbass driver. The Q of the vent modes is quite high and not responsive to electronic filtering, since notching-out the vent also notches-out the woofer response, not something we want to do. A common trick in some stand-mount speakers is a rear-mounted vent, but I've found speakers like this can be very "touchy" about room location - again, the vent is a compact point-source at the frequencies where the vent is active. What the listener hears is the vector sum of the physical vent, along with the floor reflection, rear-wall reflection, and side-wall reflection. The phase angles between the real vent and these reflections can lead to large deviations in response at the listening position.

Although the vent is our friend, it's also our enemy. It's good to reduce woofer excursion at the box frequency, but it's not good to have it rise very rapidly below the box frequency. The organ-pipe modes are very undesirable and quite audible once you start recognizing the coloration for what it is (put your ear next to the vent and you've trained yourself to hear the coloration). The tiny point-source is also troublesome - it's much smaller than the woofer, so moving the speaker an inch or so will change the phase relation between the physical source and the first three images. If the speaker were in free air, we wouldn't be concerned about the images, but in all real rooms, they are significant.

A resistive-vent system trades off bass extension (RV systems have F3's similar to closed-boxes, which is why the AES Journal did not publish the section in Richard Small's doctoral thesis on RV alignments) for a number of subtle improvements. Since the inductance of the vent is partially replaced by a resistance (in practice, the vent is lined with felt, or is a labyrinth with many folds), the vent tuning is broadened, and output is reduced. The electroacoustic system is not as critically dependent on amplifier damping (and driver Q) for flat response. If the opening of the vent is large, and against a room boundary (the floor), speaker location is not as critical.

The idea is to decrease sensitivity of the alignment to room location, driver Q, and amplifier damping factor. A QB3 alignment is not quite as critical as a C4, but is still very sensitive to these three parameters. A BL4 is somewhat better, but vent coloration and room location are not improved. A large-aperture (for example, 25" wide by 2" high) vent that is adjacent to the floor, and lined with felt throughout, takes the system closer to a resistive-vent alignment, at the expense of F3 extension. I don't consider that a loss, since the F3 extension of a classical vented system is quite sensitive to room location - bass will be very deep in some locations, but getting the other speaker to have a similar extension might be difficult.

If F3 extension is sensitive to room location, then the user has to make an awkward choice between bass-matching the left and right speakers, and the best-sounding locations for the midband (500 Hz to 5 kHz). It is unlikely the best-bass locations and best-imaging locations will be the same - this is the problem faced by nearly all owners of stand-mounted 2-way systems.

Ideally, a system should have multiple subwoofers spaced around the room, which gets around the best-bass versus best-imaging problem. Just position the speakers for best image quality, and let the multiple subwoofers handle everything below 60~80 Hz.

Failing that, though, I think it's a good idea to have a 2-way system that is reasonably free of vent coloration, and is not as sensitive to amplifier damping, driver Q, and vent location. A resistive-vent system can be useful for reducing these sensitivities, but others might argue that it is simpler to use a (2nd-order) closed-box system and set aside all the troubles with vents.

Failing that, though, I think it's a good idea to have a 2-way system that is reasonably free of vent coloration, and is not as sensitive to amplifier damping, driver Q, and vent location. A resistive-vent system can be useful for reducing these sensitivities, but others might argue that it is simpler to use a (2nd-order) closed-box system and set aside all the troubles with vents.

ok. so then do you go with a "properly aligned" 2nd order box, or is a "borderline" Qes woofer (~0.51) in a closed box suitable?

Nothing complicated about a closed-box: just select an overall system Q in the 0.9 (very slight peak but better power-handling and smaller box) to 0.65 (best group-delay at the expense of decreased power-handling and larger box) range. That is a subjective trade-off; lower IM distortion can easily trump better group-delay performances, since lower distortion actually sounds like "speed" to most of us.

In principle, closed, vented, and resistive-vent box should sound nearly identical (given the same woofer), except for different F3 cutoffs and somewhat different IM distortion at the bottom of the working range. The woofer is in the piston band (flat response), and from 100 Hz on up, they really should be identical, no differences at all.

That's in principle. In practice, I hear quite different bass quality from closed, vented, resistive-vent, and TL systems, in the range well above cutoff. Why? I'm not entirely sure, except for the second-order kind of effects mentioned in the previous posts - dynamic variation in system Q, zeroes at very low frequencies leading to the VC moving slightly back and forth in the gap, etc. Of all the systems I've personally designed (too many variables assessing what other people do), my favorites are TL's and resistive-vent systems, with classical vented some distance behind, and closed-box my least favorite.

That's purely subjective, but I don't discount that. There's no point in designing something and not liking the result. The personal goal is a system I enjoy listening to.

confirming your "favorites", the old jensen transflex design has become the epitome of subwoofer performance in contemporary times, especially for a portable type of stage system. while i've never heard one of these and the program material available to make one shine is probably slim (except for connecting an audio generator to the pre-amp inputs), i'm certain the sound is quite something! i have an EV T18 that does a pretty good job of growling the lows, but of course nothing compared to the transflex. i remember "discovering" this design while i was in college (ages ago). i copied the article and believe it was scanned and archived.

however, there is a 2 way system that some have said has no rivals. it is closed box. i've never heard it. but this link shows the simple design. of course this is not a professional stage speaker. don't know if the driver employed a "high compliance" design.

I'd argue also, that classical vented systems can be improved quite a bit with the details- as you mentioned, lining a vent with felt can work, but a low-loss foam insert in conventional vents can be quite effective too (based upon subjective impressions of a single experiment, so get out the saltshaker boys). Likewise staggering vent length, flaring ends, etc, all refine the vent system and get it closer to ideal.

In the example shown, the foam is just a square piece folded into a U-shape and placed into the vent, extending past the inner boundary but not into the outer vent flare. An inner flare would be a good inclusion as well.

This is the system I'm using in my vented cabs, with an alignment that is more like an overdamped EBS than anything else- a slight knee but significantly bigger cab and than QB3 or C4, and tuning about where C4 would fall. (4.5 cu ft tuned to 37 or so, eminence magnum 15HO)